Octane Ratings: higher = better performance, right?

A lot of confusion, mystery, and myth surround the simple question of what grade of fuel to use, what octane ratings mean, what detonation is, and the value of racing fuels. Part of the reason for that is the marketing that oil companies have done for over 70 years, in which they have promoted high octane fuels as an easy avenue to increase performance, when in fact, this is a very much backwards perspective on the matter. It is engines originally built for higher performance that need it. Let’s see if we can clear up some of the misinformation!

Click on to page two where we'll start with "Detonation: what is it?"

Detonation: what is it?

Gasoline engines are designed to burn gasoline. They are not designed to “explode” or detonate it. There’s a huge difference. During normal operation, air and fuel is introduced into the combustion chamber, compressed so as to improve the force of expansion as it burns, then near top dead center, it is ignited by the ignition system. What is supposed to happen is that the fuel immediately adjacent to the spark plug is ignited, and as it burns it ignites the fuel surrounding it and so on outward from there very much like a wave of flame. It’s like making a circle of match heads on top of a table and dropping a lit match in the middle (or anywhere else in the circle). The flame will spread rapidly, but never instantly.

Detonation is different. Detonation is the simultaneous ignition of all of the unburned fuel in the cylinder at the time it occurs, all at once. Normal ignition is a heavy, sudden push that continues over 90 or more degrees of crank rotation; detonation is like hitting the piston with a hammer instead of pushing it, and often has the effect you would expect from doing that kind of thing.

Up next: Why and how does detonation happen?

Why and how does detonation happen?

Detonation occurs when the heat and pressure in the cylinder rise to the point where the gasoline is ignited by just that, and not by contact with a flame. Once that level of heat is reached, it affects all of the fuel that remains to be burned equally and at the same time, rather than progressively, as a flame traveling from droplet to droplet of the fuel mist, and all of it ignites in the same instant, creating insanely high spikes of pressure that can and do cause horrendous damage.

In practice, the causes are one or more of excessive heat in the combustion chamber that allows temperatures to rise too high during compression, timing that is too far advanced, allowing the flame to build too much pressure before it can effectively turn the crank, hot spots in carbon deposits that ignite the fuel early, or simply a fuel the does not have sufficient resistance to being ignited by heat and pressure alone. That is, a fuel with an insufficient octane number.

Most of the time, what happens is that ignition takes place normally, and the flame front begins to spread over the combustion chamber as intended. But, as the flame spreads, heat and pressures in the chamber rise until they reach the point where the fuel can no longer tolerate them, the remaining fuel from the previous intake cycle detonates, often resulting in an audible “ping”. It can produce a sound reminiscent of having filled the top end with marbles or something, and can be quite destructive if it happens early enough in the power stroke. The correction, assuming that the engine is timed and jetted appropriately is to increase the resistance of the gasoline to being ignited by sources other than open flame. That means, a fuel with a high enough octane number.

That’s the form detonation usually takes, and at that level, in which only a part of the whole fuel charge is detonated instead of burned, it isn’t nearly as harmful as it can get. In the extreme, where the gasoline is pre-ignited, that is, ignited earlier than intended by an overheated engine, glowing carbon, etc., a larger percentage, or even all of the fuel may be detonated at once, and the results can be catastrophic.

Up next: What are the factors that influence normal combustion?

What are the factors that influence normal combustion?

Compression, ignition advance, and the fuel’s tolerance of them. Compression is simple to understand. Compression creates heat, and more compression creates more heat. Possibly too much more.

Ignition timing is a little more complicated, but still pretty easy to understand. In a piston engine, there is a mechanical “sweet zone” in the rotation of the crankshaft that goes from just after top dead center (TDC) to around 90 degrees after top dead center (ATDC). In this zone pressure on top of the piston does the most efficient job of turning the crank. It’s also important here to note that throughout this zone of rotation, the combustion chamber volume is constantly expanding. Gasoline of any kind or blend burns at nearly the same rate under pressure regardless of the situation.

Now, picture the piston having just past TDC and starting down the bore. If you light the fuel now, it will take a certain amount of time for it to develop a significant amount of pressure to really do anything about pushing the piston down. While it’s trying to build pressure, the piston is moving away from it at the same time, and little is gained. So the timing is set to ignite the fuel in advance of TDC. This allows the burning fuel to build up a meaningful amount of “push” by the time the piston starts down, and ideally burning most of it within the “sweet zone”. Since the engine will speed up, but the fuel burn won’t, the faster the engine spins, the more advance it needs as it picks up speed.

But if the fuel is ignited too early, the pressure and heat may reach critical levels before the combustion chamber volume has begun to enlarge adequately, and the portion of the fuel that lies in front of the advancing flame may then detonate.

Octane is the fuel’s detonation resistance.

And that’s all it is. Gasoline is a blend of several hydrocarbon solvents, among them toluene, benzene, heptane, and octane, plus a number of less active ingredients designed to do things other than add to the fuel’s energy levels. The number “100 octane” is based on the detonation resistance of 100% iso-octane. When a fuel is labeled “95 octane”, it resists detonation under pressure as well as a blend of gas consisting simply of 5% n-heptane and 95% iso-octane.

Octane number indicates ONLY this resistance to detonation. High octane gas does not burn hotter, colder, easier, harder, cleaner, dirtier, or with any more or less power because of the octane number. Differences such as any of these other fuel characteristics that actually do occur are the result of the overall fuel blend used for that particular gasoline, and it is both possible and common to find major differences in these qualities in different gasolines that all have the same octane rating. Race gas is a perfect example of this, as we will see later.

Up next: Why can I use 91 octane when my manual says I need at least 95?

Why can I use 91 octane when my manual says I need at least 95?

There are three different rating systems used to find the octane number of a fuel. The oldest is the Research Method. This method uses a special test engine with a variable compression ratio to compare the relative detonation resistance of fuels with equivalent heptane/octane mixes.

A newer method called the Motor Octane method also uses a test engine, but runs at 900 RPM instead of 600 as in the Research Method, and uses higher temperatures and variable timing to compare fuels. It is considered a more accurate gauge of how gasoline will perform in modern engines than is the Research Method, but it’s rarely used in any kind of advertising because the rating numbers tend to run from about 8 to 12 points lower than the ratings arrived at with the Research engine. A fuel rated 100 Research Octane Number (RON) will only post up a best of 90-92 Motor Octane Number (MON), in spite of the fact that they have very close to the same real detonation resistance regardless of the test method. But oil companies are much more likely to promote their products by quoting RON than MON, if you let them, because it comports with all those marketing myths they’ve been selling all these years. This is where the third rating method comes in.

In an effort to reduce consumer confusion and promote some level of consistency, the US Government requires that the average octane number achieved by both methods be posted on gas pumps and be called the “Anti-Knock Index”. You see it as “R+M/2” on the pump. So when your manual says you need 95 octane, and your bike is from Europe or Japan, you’re being quoted Research Octane Number. The equivalent Motor Octane number would be about 86, and the average would be 90-91, so that’s what you would look for at the gas pump.

Up Next: So, do I need race fuel?

So, do I need race fuel?

If you can buy pump gasoline that meets the minimum octane requirements of your engine, you don’t need race gas or octane boosters to raise the octane number any higher. Your engine will run detonation-free on any gas that rises to that level, and paying any money out to run the octane rating up any higher than that is just a pure waste.

There are, or may be, several other reasons to improve on the pump gas you find in your particular area. A lot of what goes into commercial automotive pump gas is there to do things other than create power, and those ingredients may be partially or completely inert as far as their contribution to the amount of power the engine can produce from it (referred to as “energy content”). Ethanol fuels are a good example. By itself, ethanol has an RON of 108, but its MON is only 88. E85 fuel is 104 RON, and only 85 MON. Furthermore, to get the same power as non ethanol gasoline, you have to burn 15-25% more of it.

Oxygenating agents are added to pump fuels to aid in the more complete burning of fuel for the purpose of reducing emissions. Oxygenates are added to race fuels as accelerants, and there is often a fairly big difference in the chemicals chosen for that job. Ethanol is an oxygenate, but it produces much less energy per volume in and of itself than most gasoline components, so it reduces the energy content. MTBE (methyl tertiary butyl ether) is an oxygenate that produces more energy when burned than ethanol, and releases more oxygen in the process, so it’s more often used in race fuels.

You can generally gain power through using race gas, but rather than a gift that keeps on giving, it’s a modification that you have to keep on paying for for as long as you want to use it, and it often requires rejetting to make the switch, so you kind of have to stay with it. The extra power IS NOT a result of the usually higher octane number, but comes from the specific blend of hydrocarbon compounds used in the formula. A number of octane increasing components such as xylene and toluene also increase the energy content since they actively contribute to the combustion event, as opposed to tetraethyl lead, with is essentially inert as a fuel component.

Incidentally, octane boosters are mostly snake oil. There are a few good ones that are available from automotive speed shops, but most of the ones you see on the shelf at the auto parts store are useless. They say they raise octane by one or two or three points, but that’s a change of 0.1 to 0.3, not 1 to 3 octane. Injector cleaner might actually be more effective.

Next: So, will more octane benefit me?

So, will more octane benefit me?

An excess of octane number beyond what your engine needs is completely harmless and has no downside except for the damage it does to your wallet. If it’s simply a question of octane, as long as you don’t ping on ordinary pump premium, you don’t need any more octane to prevent detonation, and preventing detonation is the only thing high octane is good for.

Have a question or comment? Post it below! You'll be helping to expand the depth & value of this topic and at the end of the day, that's my goal!

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I recommend nuking the original article and replacing it with this one.

Thanks!

Someone mentioned that I am nitpicking about the terminology, but I am just trying to make it clearer. I have spent the last 14yrs working for NGK Spark Plugs and deal with these things everyday. If the purpose of these articles is to educate, then we should provide correct concepts and terminology.

Also, to address Slartidbartfast's comment about "pre-ignition by itself means nothing" - not exactly sure what you mean, but that doesn't sound right to me. Knock and pre-ignition are two different phenomenon. Severe knock can raise the cylinder temperature enough to heat up components, in turn causing pre-ignition. But pre-ignition can also occur without knock. For example, when someone uses a spark plug with too hot of heat range. The ceramic on the firing end overheats when heat cannot dissipate to the cylinder head fast enough. The hot ceramic will turn into the source of ignition - before the spark occurs. Furthermore, if there is no knock, you usually don't hear any noise during pre-ignition. The cylinder pressure and temperature rise so fast that it only takes a few seconds of pre-ignition to melt a plug or piston. On the other hand, I have heard engines run all day long, full load, max rpm, with heavy knock and they survive without failing, because pre-ignition does not occur. So - knock and pre-ignition are related, but are two distinct forms of abnormal combustion. And they can occur independently.

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You can't make a racing fuel that has the best of everything, but you can produce one that will give your particular engine the most power. This is why we produce different fuels for different applications. The key to getting the best racing gasoline is not necessarily buying the fuel with the highest octane, but getting one that is best suited for your engine.

1. OCTANE – This is simply the rating of a fuel’s ability to resist detonation and/or preignition. Octane is rated in Research Octane Numbers (RON), Motor Octane Numbers (MON), and Pump Octane Numbers (R+M/2). Pump Octane Numbers are what you see on the yellow decal at the gas stations and represents an average of RON and MON. VP reports MON ratings because this method tests a fuel’s performance under a heavier load than the RON method, thus better simulates racing conditions. Most other companies use RON because it sounds better in marketing messages. Don't be fooled by high RON numbers or an average—MON is the most relevant for a racing application. However, a fuel’s ability to resist preignition is more than just a function of octane.

2. BURNING SPEED - The speed at which fuel releases its energy. In a high-speed internal combustion engine, there is very little time (real time - not crank rotation) for the fuel to release its energy. Peak cylinder pressure should occur around 20° ATDC. If the fuel is still burning after this, it is not contributing to peak cylinder pressure, which is what the rear wheels see.

3. ENERGY VALUE - An expression of the potential in the fuel. The energy value is measured in BTUs per pound, not per gallon. The difference is important. The air:fuel ratio is in weight, not volume. Remember, this is the potential energy value of the fuel. This difference will show up at any compression ratio or engine speed.

4. COOLING EFFECT: The cooling effect on fuel is related to the heat of vaporization. The higher the heat of vaporization, the better its effect on cooling the intake mixture. This is of some benefit in a four-stroke engine, but can be a big gain in two-stroke engines.

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Av Gas , due to the aromatics, iso octane and good old Pb/ organics, is what make is wonderful fuel. Planes go high, hence reduced air pressure. Garbage fuel would vapour lock!!! Yes, to make it worth while, advance ignition timing and raise static compression. Even in a 25 hp 2 stroke outboard, there was more power, with no change to tuning. Aromatics have more BTU,s than most things in Gas.

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OCTANE – This is simply the rating of a fuel’s ability to resist detonation and/or preignition...

...However, a fuel’s ability to resist preignition is more than just a function of octane.

Looks like even the VP guys don't fully understand the difference between knock and pre-ignition...

From VP website quote: "This is simply the rating of a fuel’s ability to resist detonation and/or preignition." It's not detonation or pre-ignition - it's the resistance to knock. Pre-ignition is also known as surface ignition - where a hot spot ignites the air-fuel mixture before the spark between the spark plug gap does. When we do pre-ignition (spark plug heat range) testing, we are always limited by knock when using lower octane fuels. In other words, knock occurs first when advancing the ignition timing to find the pre-ignition threshold. If we use high octane fuel (ex. 103 RON), then we can usually avoid knock and heat the spark plug up enough to cause pre-ignition by advancing the timing.

Another quote from VP website: "However, a fuel’s ability to resist preignition is more than just a function of octane." It's not the ability to resist pre-ignition - it's the ability to resist knock. Low octane fuel doesn't directly cause pre-ignition. Knock occurs first if the spark plug heat range is appropriate. Severe, high levels of knock can eventually overheat combustion chamber components enough to cause pre-ignition (surface ignition) - but the knock comes first.

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To prevent destructive knock or ping you would dither ignition timing based on load, temperature, and octane like modern automotive engines do. Automotive ECMs can also indirectly adjust the effective compression ratio by adding inert exhaust gas with external valves (EGR) or by playing with valve timing.

I don't know about modern injected bikes but dinosaurs like my XRs don't have any of that stuff so I use the highest octane fuel I can get.

Not because of some illusion of greater power but simply because you just cannot hear spark knock or pinging on an air cooled bike with a pipe.

It's worth the 20 cents a gallon for the peace of mind that my piston will never look like some midget has been hammering on it with a welding hammer

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"Detonation occurs when the heat and pressure in the cylinder rise to the point where the gasoline is ignited by just that, and not by contact with a flame". - Did you mean spark?

"as a flame traveling from molecule to molecule" - that is far from how physics works.

No, I meant flame. The gasoline in the combustion chamber does not collect itself into a single contiguous blob. The air/fuel mix is a mass of air with separate droplets of gasoline suspended in it. The spark plug ignites only the gasoline droplet(s) that lie immediately adjacent to the plug, and from there, each burning droplet ignites its neighbor, and so on.

Using the phrase, "molecule to molecule" may have been chemically incorrect, but if you understood from that that the flame proceeds as an expanding wave rather than a single instantaneous event, the phrase served it intended purpose correctly.

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Looks like even the VP guys don't fully understand the difference between knock and pre-ignition...

From VP website quote: "This is simply the rating of a fuel’s ability to resist detonation and/or preignition." It's not detonation or pre-ignition - it's the resistance to knock. Pre-ignition is also known as surface ignition - where a hot spot ignites the air-fuel mixture before the spark between the spark plug gap does. When we do pre-ignition (spark plug heat range) testing, we are always limited by knock when using lower octane fuels. In other words, knock occurs first when advancing the ignition timing to find the pre-ignition threshold. If we use high octane fuel (ex. 103 RON), then we can usually avoid knock and heat the spark plug up enough to cause pre-ignition by advancing the timing.

Another quote from VP website: "However, a fuel’s ability to resist preignition is more than just a function of octane." It's not the ability to resist pre-ignition - it's the ability to resist knock. Low octane fuel doesn't directly cause pre-ignition. Knock occurs first if the spark plug heat range is appropriate. Severe, high levels of knock can eventually overheat combustion chamber components enough to cause pre-ignition (surface ignition) - but the knock comes first.

Pre-ignition can be caused by insufficient octane number. It's a fairly rare occurrence because in order for this to happen, the heat generated by compression has to rise to the point of igniting the fuel before anything else does it, in the manner of a Diesel engine, and the octane number has to be very far off to accomplish that. "Anything else" might include carbon deposits glowing from compressive heating, or another indirect source of ignition from leftover or accumulated heat that happens prior to the intended ignition point. Pre-ignition essentially produces detonation by moving the effective ignition timing to a point that is too early.

No, I meant flame.
(...)
Using the phrase, "molecule to molecule" may have been chemically incorrect, but if you understood from that the the flame proceeds as an expanding wave rather than a single instantaneous event, the phrase served it intended purpose correctly.

The flame is started by the spark. I understand that the combustion process can be broken up into phases, but your terminology is confusing to me.
I think that in a technical explanation, one should be expected to be clear and precise...
Maybe your article is good, but some things in it give me pause.

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Someone mentioned that I am nitpicking about the terminology, but I am just trying to make it clearer. I have spent the last 14yrs working for NGK Spark Plugs and deal with these things everyday. If the purpose of these articles is to educate, then we should provide correct concepts and terminology.

Also, to address Slartidbartfast's comment about "pre-ignition by itself means nothing" - not exactly sure what you mean, but that doesn't sound right to me. Knock and pre-ignition are two different phenomenon. Severe knock can raise the cylinder temperature enough to heat up components, in turn causing pre-ignition. But pre-ignition can also occur without knock. For example, when someone uses a spark plug with too hot of heat range. The ceramic on the firing end overheats when heat cannot dissipate to the cylinder head fast enough. The hot ceramic will turn into the source of ignition - before the spark occurs. Furthermore, if there is no knock, you usually don't hear any noise during pre-ignition. The cylinder pressure and temperature rise so fast that it only takes a few seconds of pre-ignition to melt a plug or piston. On the other hand, I have heard engines run all day long, full load, max rpm, with heavy knock and they survive without failing, because pre-ignition does not occur. So - knock and pre-ignition are related, but are two distinct forms of abnormal combustion. And they can occur independently.

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Pre-ignition can be caused by insufficient octane number. It's a fairly rare occurrence because in order for this to happen, the heat generated by compression has to rise to the point of igniting the fuel before anything else does it, in the manner of a Diesel engine, and the octane number has to be very far off to accomplish that. "Anything else" might include carbon deposits glowing from compressive heating, or another indirect source of ignition from leftover or accumulated heat that happens prior to the intended ignition point. Pre-ignition essentially produces detonation by moving the effective ignition timing to a point that is too early.

I think it would be clearer to put it this way: Low octane fuel can cause knock. (Knock occurs after the spark occurs.) Excessive knock can heat up the combustion chamber components so much that hot spots can eventually cause pre-ignition. (Pre-ignition means that the air-fuel mixture is ignited before the spark occurred at the spark plug gap.) That's the order of how it happens. In the automotive engineering world, we don't use the word detonation to describe anything like knock or pre-ignition. We really don't use the word detonation to describe anything...

The line is getting blurred a little as there are recent findings that oil mist and carbon in the cylinder can also cause a phenomenon called "Megaknock". (This usually only occurs in turbocharged engines.) Megaknock is similar to pre-ignition in the fact that the cylinder pressure starts to rise quickly from combustion before the spark event. There is also another phenomenon called "crevice volume pre-ignition" that occurs when hot gases are trapped in the area between the spark plug center electrode and the plug shell. These gases can ignite the air-fuel mixture before the spark event, with a visual that looks kind of like a pre-chamber spark plug (flame blasting out of the plug firing end) when viewed with an in-cylinder camera.

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I think it would be clearer to put it this way: Low octane fuel can cause knock. (Knock occurs after the spark occurs.) Excessive knock can heat up the combustion chamber components so much that hot spots can eventually cause pre-ignition. (Pre-ignition means that the air-fuel mixture is ignited before the spark occurred at the spark plug gap.) That's the order of how it happens. In the automotive engineering world, we don't use the word detonation to describe anything like knock or pre-ignition. We really don't use the word detonation to describe anything...

The line is getting blurred a little as there are recent findings that oil mist and carbon in the cylinder can also cause a phenomenon called "Megaknock". (This usually only occurs in turbocharged engines.) Megaknock is similar to pre-ignition in the fact that the cylinder pressure starts to rise quickly from combustion before the spark event. There is also another phenomenon called "crevice volume pre-ignition" that occurs when hot gases are trapped in the area between the spark plug center electrode and the plug shell. These gases can ignite the air-fuel mixture before the spark event, with a visual that looks kind of like a pre-chamber spark plug (flame blasting out of the plug firing end) when viewed with an in-cylinder camera.

The first couple of sentences basically just restate what I said. Pre-ignition is caused by any condition that ignites the fuel prior to the intended ignition point.

However, while you may not use the word "detonation", that is precisely what "knock" is. "Knock" is the lay term for the audible sound fuel makes as it detonates, rather than burns. Pre-ignition causes it for the same reason that too much spark advance does.

The earlier in the burn the detonation happens, the worse it is because more fuel is involved. Your "mega-knock" condition is simply that; detonation occurring very early on in the burn so that it involves a larger portion of the fuel charge. Detonation events have been known to produce dramatic, even spectacular failures, bending connecting rods, breaking pistons and cranks, and blowing whole cylinders off engines. It is often worse on supercharged engines since the whole idea of supercharging by any means is to force the engine to process a larger quantity of fuel.

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